Market dynamics are changing the way refiners are looking at their alkylation units. More restrictive environmental controls such as Tier 3 motor vehicle emission and fuel standards and CAFE regulations continue to drive the need for additional alkylation capacity in North America. Refiners have the opportunity to create additional value by increasing alkylation capacity using currently available olefins. Processing so-called opportunity feedstocks, such as propylene and C5 olefins, can significantly enhance revenue for the refiner with an optimised processing strategy.

Propylene alkylation will become increasingly attractive as refinery propylene prices face substantial downward pressure as additional propylene production capacity is brought on line. Alkylation of fluid catalytic cracking (FCC) C5 olefins upgrades low-value and widely available butanes to gasoline. This upgrade also has the positive effect of reducing gasoline pool RVP. This article will address the use of field butanes to manufacture on-purpose alkylate.

Although alkylation of propylene and amylene has been long practised, technical challenges have constrained the potential economic benefits to the refiner, and thus limited a broader acceptance of these opportunity feedstocks. Higher acid consumption rates, additional waste streams, environmental and safety concerns, lower alkylate yield, lower alkylate octane value, and increased corrosion rates are challenges facing refiners when evaluating the merits of propylene and amylene alkylation via conventional sulphuric acid alkylation units. CB&I’s portfolio of refining technologies provides solutions to these as well as other challenges confronting today’s refiner. Here, we address several innovative strategies for alkylating opportunity feedstocks via CB&I’s CDAlky technology, thus creating value for the refiner.

Road octane requirements are expected to increase in the US largely due to the pressures of meeting the latest CAFE regulations. Furthermore, gasoline pool octane losses resulting from Tier 3 implementation must be replaced. Levels of US gasoline exports have also increased more than two-fold since 2010, with most export grades having a RON value in the 91 to 95 range.1 To help meet this challenge, alkylate demand is expected to increase because of its excellent blend characteristics, namely high octane value, low vapour pressure, absence of olefins or aromatics, and very low sulphur content. With more alkylate available for blending, Tier 3 strategies can be optimised and potentially streamlined.

The retail value of octane in the US has increased consistently since 2010,2 and this trend is expected to continue. At the wholesale level, the trend is somewhat different, with the incremental cost of octane following crude prices quite closely, particularly Brent. US refiners have assets currently in place to meet the growing demand for octane, albeit at a higher cost; for example operating marginal catalytic reformers at higher severities or higher utilisation rates.1 Figure 1 plots the yearly average US Gulf Coast alkylate price3 as a surrogate for octane value, along with Brent crude. The trend is unmistakably similar.

Currently, worldwide alkylate production capacity stands at approximately 2.1 million b/d. Nearly 60% of this capacity is located in North America,4 with alkylate being produced by the reaction of isobutane with light olefins, primarily C4, from FCC units. The mixture of multi-branched, gasoline range hydrocarbons thus formed is an excellent clean gasoline blending stock.

CDAlky technology
The ability to process, and benefit from, opportunity feedstocks as presented in this article can be attributed to the innovative design of the CDAlky reaction system (see Figure 2). The reactor design not only allows the refiner to economically break the low temperature barrier and achieve enhanced product quality, but also eliminates the complex alkylate post-treatment section which can cause downstream corrosion problems. These step-changes in process technology have been the only real breakthroughs in sulphuric acid alkylation for over 50 years.

Propylene alkylation
Refiners without either alternate processing options or an outlet to market for their surplus refinery grade propylene (RGP) have practised propylene alkylation for some time. With additional propylene production coming on line in the US via FCC and propane dehydrogenation, downward pricing forces due to surpluses may create opportunities for revenue enhancement.

Propylene alkylation requires more severe reaction conditions, particularly with sulphuric acid processes, since the reaction rate is slow compared to C4 and C5 olefins. It is also well known that alkylate octane and quality, acid consumption, and yield are negatively affected when processing propylene. To increase the reaction rates and hence improve alkylate product quality and yield, the propylene feedstock should be segregated from the C4 and C5 olefins, and fed at high concentration to alkylation reactors. The propylene reaction system will operate at a higher temperature, greater isobutane to olefin ratio, and acid strength, but will operate at a lower space velocity compared to C4 and C5 olefin alkylation.

With conventional sulphuric acid alkylation technology, high concentrations of propyl sulphates are brought about by elevated propylene concentrations. As a result, acid consumption becomes extremely difficult to control, and acid runaway conditions can become more likely. First, acid consumption increases to the extent that maintaining a constant acid strength in the reaction section becomes extremely challenging. Secondly, the presence of propyl sulphates at high concentrations will influence the physical properties of the acid emulsion. Not only is the ‘mixing’ affected by an increased emulsion viscosity, but also by the emulsion foaming/frothing properties. These factors explain why conventional alkylation reactors are limited in the amount of propylene they can tolerate. CDAlky reactors, which employ no moving parts, are the first devices able to safely process 100% propylene on a total olefin basis.

Since lower alkylate octane value and higher acid consumption are typically associated with propylene alkylation, this practice should be considered as a portion of an overall refinery olefin processing strategy to ensure viable economic performance. Due to its feedstock processing flexibility and ability to cascade hydrocarbon, CDAlky can present a range of possibilities for optimisation, adding value for refiners.

An integrated, optimised solution minimising the shortcomings described earlier is shown in Figure 3.

In the processing scheme shown in Figure 3, the olefin feeds are staged. The CDAlky reactor processing propylene is oriented upstream of the reactor processing butylenes. This arrangement enables operating temperatures to be optimised; that is, low temperature for the butylene unit and a higher temperature for the propylene unit. One important factor differentiating CDAlky from conventional sulphuric acid alkylation technology is its ability to process up to 100% propylene, on an olefin basis, which adds value to the refiner’s bottom line.